US 7076015 B2 Abstract A method is provided of detecting one of a set of preamble sequences in a spread signal. The method includes: (a) correlating the received spread signal with sequences of a first orthogonal Gold code (OGC) set in accordance with a first fast transform to provide a preamble signal; (b) correlating the preamble signal with the set of preamble sequences in accordance with a second fast transform to generate a set of index values; (c) forming a decision statistic based on the set of index values: and (d) selecting, as the detected one of the set of preamble sequences, a preamble sequence corresponding to the decision statistic. Step (c) comprises the steps of: 1) forming an initial decision statistic based on the relative maximum index of the set of index values; 2) selecting the signal generated by the preamble sequence combined with the preamble signal corresponding to the initial decision statistic; 3) adjusting, in one or more of amplitude and phase, the signal selected in step 2); and 4) forming the decision statistic based on the adjusted signal.
Claims(20) 1. A method of detecting one of a set of preamble sequences in a spread signal comprising the steps of:
(a) correlating a received spread signal with sequences of a first orthogonal Gold code (OGC) set in accordance with a first fast transform to provide a preamble signal;
(b) correlating the preamble signal with the set of preamble sequences in accordance with a second fast transform to generate a set of index values, wherein each index value of the set of index values corresponds to a one preamble sequence of the set of preamble sequences;
(c) forming a decision statistic based on the set of index values; and
(d) selecting, as the detected one of the set of preamble sequences, a preamble sequence corresponding to the decision statistic;
wherein step (c) comprises the steps of:
1) forming an initial decision statistic based on the a relative maximum index of the set of index values;
2) selecting a signal generated by the preamble signal combined with the preamble sequence corresponding to the relative maximum index of the initial decision statistic;
3) adjusting, in one or more of amplitude and phase, the selected signal in step 2); and
4) forming the decision statistic based on the adjusted selected signal.
2. The invention as recited in
1) multiplying the received spread signal with a first sequence vector and a forward permutation vector to generate a permuted sequence signal, wherein:
the first OGC set is generated from the first sequence vector and a cyclic shift matrix of a second sequence vector, and the forward permutation vector maps between i) the cyclic shift matrix of the second sequence vector and ii) a matrix of Walsh-Hadamaard sequences; and
2) applying a fast Hadamaard transform to the permuted sequence signal to generate a set of correlated signals, the preamble signal selected as one of the set of correlated signals based on a predetermined decision criterion.
3. The invention as recited in
for step (b), the set of preamble sequences are selected from a second OGC set formed from first and second sequence vectors, the second OGC set generated from the first sequence vector and a cyclic shift matrix of the second sequence vector; and wherein
the second fast transform is a fast orthogonal Gold code transform (FOGT) comprising the steps of
1) multiplying the preamble signal with the first sequence vector and a forward permutation vector to generate a permuted preamble signal, the forward permutation vector mapping between i) the cyclic shift matrix of the second sequence vector and ii) a matrix of Walsh-Hadamaard sequences, and
2) applying a fast Hadamaard transform to the permuted preamble signal to generate the set of index values.
4. The invention as recited in
5. The invention as recited in
6. The invention as recited in
7. The invention as recited in
8. A method of detecting one of a set of preamble sequences in a spread signal comprising the steps of:
(a) correlating a received spread signal with a set of orthogonal sequences to provide a preamble signal;
(b) correlating the preamble signal with one or more preamble sequences of an orthogonal Gold code (OGC) set in accordance with a fast transform to generate a set of index values, wherein each index value of the set of index values corresponds to a one preamble sequence of the set of preamble sequences;
(c) forming a decision statistic based on the set of index values; and
(d) selecting, as the detected one of the set of preamble sequences, a preamble sequence corresponding to the decision statistic;
wherein step (c) comprises the steps of:
1) forming an initial decision statistic based on a relative maximum index of the set of index values;
2) selecting a signal generated by the preamble signal combined with the preamble sequence corresponding to the relative maximum index of the initial decision statistic;
3) adjusting, in one or more of amplitude and phase, the selected signal in step 2); and
4) forming the decision statistic based on the adjusted selected signal.
9. The invention as recited in
for step (b), each preamble sequence is selected from the OGC set formed from first and second sequence vectors, wherein the OGC set is generated from the first sequence vector and a cyclic shift matrix of the second sequence vector; and wherein
the fast transform is a fast orthogonal Gold code transform (FOGT) comprising the steps of
1) multiplying the preamble signal with the first sequence vector and a forward permutation vector to generate a permuted preamble signal, the forward permutation vector mapping between i) the cyclic shift matrix of the second sequence vector and ii) a matrix of Walsh-Hadamaard sequences; and
2) applying a fast Hadamaard transform to the permuted preamble signal to generate the set of index values.
10. A preamble detector for detecting one of a set of preamble sequences in a spread signal, the preamble detector comprising:
a first correlator correlating a received spread signal with sequences of a first orthogonal Gold code (OGC) set in accordance with a first fast transform to provide a preamble signal;
a second correlator correlating the preamble signal with the set of preamble sequences in accordance with a second fast transform method to generate a set of index values, wherein each index value of the set of index values corresponds to a one preamble sequence of the set of preamble sequences;
a circuit forming a decision statistic based on the set of index values; and
a selector selecting, as the detected one of the set of preamble sequences, a preamble sequence corresponding to the decision statistic;
wherein the circuit forming the decision statistic comprises:
a first magnitude detector forming an initial decision statistic based on a relative maximum index of the set of index values;
a signal selector selecting a signal generated by the preamble signal combined with the preamble sequence corresponding to the relative maximum index of the initial decision statistic;
a coherent detector adjusting, in one or more of amplitude and phase, the selected signal from the signal selector; and
a second magnitude detector forming the decision statistic based on the adjusted selected signal.
11. The invention as recited in
the first correlator comprises:
a multiplier multiplying the received spread signal with the first sequence vector and the forward permutation vector to generate a permuted sequence signal; and
a combiner applying a fast Hadamaard transform to the permuted sequence signal to generate a set of correlated signals, the preamble signal selected as one of the set of correlated signals based on a predetermined decision criterion.
12. The invention as recited in
the set of preamble sequences is selected from a second OGC set formed from first and second sequence vectors, the second OGC set generated from the first sequence vector and a cyclic shift matrix of the second sequence vector; and the second fast transform is a fast orthogonal Gold code transform (FOGT); and wherein:
the second correlator comprises:
a multiplier multiplying the preamble signal with the first sequence vector and a forward permutation vector to generate a permuted preamble signal, the forward permutation vector mapping between i) the cyclic shift matrix of the second sequence vector and ii) a matrix of Walsh-Hadamaard sequences, and
a combiner applying a fast Hadamaard transform to the permuted preamble signal to generate the set of index values.
13. The invention as recited in
14. The invention as recited in
15. The invention as recited in
16. The invention as recited in
17. The invention as recited in
18. A preamble detector for detecting one of a set of preamble sequences in a spread signal comprising the:
a first correlator correlating a received spread signal with a set of orthogonal sequences to provide a preamble signal;
a second correlator correlating the preamble signal with one or more preamble sequences of an orthogonal Gold code (OGC) set in accordance with a fast transform to generate a set of index values, wherein each index value of the set of index values corresponds to a one preamble sequence of the set of preamble sequences;
a circuit forming a decision statistic based on the set of index values; and
a selector selecting, as the detected one of the set of preamble sequences, a preamble sequence corresponding to the decision statistic;
wherein the circuit forming the decision statistic comprises:
a first magnitude detector forming an initial decision statistic based on a relative maximum index of the set of index values;
a signal selector selecting a signal generated by the preamble signal combined with the preamble sequence corresponding to the relative maximum index of the initial decision statistic;
a coherent detector adjusting, in one or more of amplitude and phase, the selected signal from the signal selector; and
a second magnitude detector forming the decision statistic based on the adjusted selected signal.
19. The invention as recited in
each preamble sequence is selected from the OGC set formed from first and second sequence vectors, wherein the OGC set is generated from the first sequence vector and a cyclic shift matrix of the second sequence vector and the fast transform is a fast orthogonal Gold code transform (FOGT); and wherein
the second correlator comprises:
a multiplier multiplying the preamble signal with the first sequence vector and a forward permutation vector to generate a permuted preamble signal, the forward permutation vector mapping between i) the cyclic shift matrix of the second sequence vector and ii) a matrix of Walsh-Hadamaard sequences; and
a combiner applying a fast Hadamaard transform to the permuted preamble signal to generate the set of index values.
20. The invention as recite in
Description This application claims priority of European Patent Application No. 99310117.9, which was filed on Dec. 15, 1999. 1. Field of the Invention The present invention relates to signal demodulation in a code-division, multiple-access (CDMA) telecommunication system, and, more particularly, to despreading and preamble detection of code sequences in a CDMA receiver. 2. Description of the Related Art In wireless, cellular, or similar code-division, multiple-access (CDMA) telecommunication systems, multiple channels are defined for up-link (user to base station) and down-link (base station to user) communications. Generally, each channel includes a spread signal synchronized to a common timing reference. A pilot signal is an unmodulated code sequence that is a transmitted throughout the system for synchronization of the various code sequences (in time and in code phase), and the pilot signal is used by the spreading (modulation) and de-spreading (demodulation) operations. Channel signals may be spread, in frequency, using pseudo-random (PN) and/or orthogonal code sequences. Signals transmitted through a channel are binary data represented by a sequence of values, each value termed a symbol. The spreading PN code sequence is also binary data represented by a sequence of values, each value called a chip. However, the width (length in time) of a chip is much less than the width of a symbol. For some systems operating in accordance with the IS-95 or wideband CDMA (W-CDMA) standards, each symbol may contain up to 256 chips. The spread signal may then be combined with an orthogonal code, such as a Walsh code (also known as a Walsh-Hadamaard sequence), to maintain relatively low to zero cross-correlation between multiple spread signals transmitted through a transmission medium. Each orthogonal code is also generated as a sequence of binary values, the width of each value equivalent to the symbol width. Channels of these CDMA telecommunication systems may either be user channels or system channels. Voice, voiceband data, or data signals are generally transmitted over user channels, while system channels are used by a base station and users to broadcast information or are randomly accessed by users or the base station to establish a connection over a channel. The channels are usually spread using the same PN code sequences, and then each is spread with uniquely assigned orthogonal code signal. A receiver may then differentiate the channels using the unique, orthogonal code signal that is assigned to each channel. Since the same PN code sequence is used for all channels, and the PN code sequence phase is readily determined with reference to the pilot signal, detection of the orthogonal code signal occurs only at the beginning of the user's session. The sessions of user channels are relatively long, so the detection process for each user occurs relatively infrequently. In addition, the base station and user have the following a priori information: 1) the connection information when initialized, 2) the orthogonal code assigned to the user channel, and 3) coarse synchronization information for the initial detection process. Commonly used channels, broadcast channels, or randomly-accessed channels, on the other hand, have bursty sessions that are relatively short, since only a small amount of information is exchanged between the user and base station, for example, to initialize a connection. In addition, the receiver has no a priori information about the code sequence used, and the potential code sequence changes relatively often. Consequently, the orthogonal code detection process for these channels may occur frequently, and systems desirably minimize the time required to perform detection. For these channels characterized by bursty sessions, a transmitter may use a preamble characterized by a signature sequence to enable rapid detection and demodulation of spread signals. For example, a burst of the Random Access Channel (RACH) in accordance with W-CDMA standards is transmitted by the mobile user to initialize a dedicated set up procedure or for transporting data across the air interface (connectionless data transfer). A typical RACH burst A preamble detector may employ the following a priori information to detect a given RACH preamble spreading (code number) and/or signature sequence. First, a user randomly selects one sequence out of a code set allocated to the specific sector. Second, the sequences are known but the number or subset available for use may vary depending on the traffic load of the sector. Third, the receiver (including the preamble detector) acknowledges multiple requests from RACH bursts occurring during the same frame offset. A CDMA receiver may employ non-coherent demodulation of the signal of a user channel. For systems conforming to the IS-95 standard, for example, the receiver may use 64-ary, non-coherent demodulation (despreading) of channels. For this case, a demodulator generates all of the possible (i.e., 64) orthogonal code sequences and combines each with the received signal. A CDMA receiver may also employ coherent demodulation of the signal of a channel. In coherent demodulation, an initial estimate of the assigned code signal for the channel is determined from the received (actual code) signal. Using the initial estimate, a signal representing the error between the actual coded signal and the transmit code signal is employed to correct amplitude and phase deviations of the actual code signal. Detection of the RACH burst preamble spans three axes. The first axis is the time axis that evaluates the energy content of hundreds of Tc/2 delay cells (Tc is the chip duration). The exact length of the uncertainty area depends on the size of the cell. The second axis is the code axis that evaluates these delay cells for all permissible spreading codes. The third axis is the frequency/phase uncertainty axis, which axis is related to the detection statistic (e.g., a non-coherent, quasi-coherent, or coherent decision statistic) used to determine the relative likelihood that the correct code with correct frequency and phase is detected. The present invention relates to detection of one of a set of preamble sequences in a spread signal where either the preamble sequence is an orthogonal code sequence, the spreading sequence of the spread signal is an orthogonal code sequence, or both the preamble and spreading sequences are orthogonal code sequences. The orthogonal code sequences may be selected from either a set of Walsh-Hadamaard sequences or a set of orthogonal Gold code (OGC) sequences. Where the orthogonal code sequences are selected from the set of Walsh-Hadamaard sequences, a detector and/or demodulator may employ a fast Hadamaard transform (FHT) method for signal correlation. Where the orthogonal code sequences are selected from the set of OGC sequences, a detector and/or demodulator may employ a fast transform method based on the method used to generate the OGC sequences, the fast transform method including the FHT transform. In accordance with an embodiment of the present invention, a preamble detector correlates the received spread signal with sequences of a first orthogonal Gold code (OGC) set in accordance with a first fast transform method to provide a preamble signal, wherein the spread signal is the preamble sequence combined with one sequence of the first OGC set. The preamble signal is then correlated with the set of preamble sequences in accordance with a second fast transform method to generate a set of index values. A decision statistic is formed based on the set of index values, and the preamble sequence corresponding to the decision statistic is selected as the detected one of the set of preamble sequences. For another embodiment, the received spread signal is correlated with a set of orthogonal sequences to provide a preamble signal, wherein the spread signal is the preamble sequence combined with one sequence of the set of orthogonal sequences. The preamble signal is correlated with one or more preamble sequences of an orthogonal Gold code (OGC) set in accordance with a fast transform method to generate a set of index values. A decision statistic is formed based on the set of index values; and the preamble sequence corresponding to the decision statistic is selected as the detected one of the set of preamble sequences. For some exemplary embodiments, either the spreading sequence or preamble sequence is selected from the OGC set formed from first and second sequence vectors, wherein the OGC set is generated from the first sequence vector and a cyclic shift matrix of a second sequence vector. The fast transform method is a fast orthogonal Gold code transform (FOGT) that comprises the following steps. First, the preamble signal is multiplied with a first sequence vector and a forward permutation vector to generate a permuted preamble signal, the forward permutation vector mapping between i) the cyclic shift matrix and ii) a matrix of Walsh-Hadamaard sequences. Second, the fast Hadamaard transform is applied to the permuted preamble signal to generate the set of index values. Other aspects, features, and advantages of the present invention will become more fully apparent from the following detailed description, the appended claims, and the accompanying drawings in which: In accordance with the present invention, a code-division, multiple-access (CDMA) preamble detector applies either a Fast Hadamaard Transform (FHT) or a Fast Orthogonal Gold-code Transform (FOGT) to the received spread signal for either of the despreading and preamble signature sequence detection processes. Implementations operating in accordance with the present invention may allow for relatively simple detection circuits. Detection of preamble sequences may allow rapid detection and demodulation of data signals in channels randomly accessed by multiple users in a CDMA telecommunication system, and such channels may be, for example, the random access channel (RACH) of systems operating in accordance with wideband-CDMA (W-CDMA) standards. For the exemplary implementations of the present invention as described below, a CDMA receiver detects the preamble part of a random access channel (RACH) burst, although the techniques as described herein may be readily extended to other CDMA applications. The RACH burst may be as shown in Each symbol of the preamble signature sequence is spread with a 256-chip code sequence from an OGC set of length 256. Consequently, 256 codes sequences are available (code numbers C1, . . . ,C256) and are allocated through the CDMA network for spreading of channels. The code numbers that a user may select are transmitted in the sector-specific broadcast channel (BCCH). The user randomly selects one of the permissible codes and one of the 16 preamble signatures, and uses the selected signature sequence to create the preamble of a RACH burst. Once the received signal is despread, preamble detection in accordance with exemplary implementations of the present invention correlates signature sequences against the despread input vector (i.e., signal samples) using fast transform techniques, and then forms a decision statistic to identify the preamble signature sequence. If coherent or quasi-coherent detection is employed, preamble detection corrects for phase or amplitude variations (if coherent or quasi-coherent detection is employed) to create the final decision statistic. Once the preamble is detected, demodulation and data detection of the message part of the RACH burst may be in accordance with techniques well known in the art. The FOGT transform is related to the FHT transform. The FOGT transform may be extended to sequences that belong to any OGC set. Orthogonal Gold code sequences may be generated as a base-2 Galois field (GF(2)) addition of selected pairs of pseudo-random code sequences (m-sequences). The length, or period, of an m-sequence is defined as N, where N is 2 The OGC set employed in accordance with the exemplary embodiment of the present invention for either spreading the preamble or the signature sequence may be generated as follows. First, the matrix X is formed as shown in equation (2):
Second, the column vector b Third, the matrix C is formed by addition over GF(2) each row of matrix S For example, each spread RACH preamble symbol may be represented by one of the rows of the 256Χ256 matrix C derived from equation (6) as given in equation (7):
It is well known in the art of communication theory that the optimum detector (or despreader) of a code sequence in AWGN may be realized as a bank of Code Matched Filters (CMFs), where each CMF may be implemented with an FIR filter matched to a corresponding cyclic shift of the code. For a random propagation delay of x chips where the propagation delay does not exceed the period of the code, the CMF matched to the sequence D A transmitted signal with additive white Gaussian noise (AWGN) that is received at a transceiver (e.g., CMF/despreader At step To calculate the permutation vector π The forward permutation vector π The method may be shown using sequences of length M=2 The permutation vector is π As would be apparent to one skilled in the art, the orthogonal matrix W is a matrix with columns representing Walsh-Hadamaard sequences of the 8Χ8 Walsh-Hadamaard matrix, and so the matrix W may be block correlated using the FHT transform. Consequently, in accordance with the exemplary embodiment, the input sequence (the combination of r and b Returning to At step For an OGC set used for spreading (e.g., OGC set of length 256) and an OGC set used for signature sequences (e.g., OGC set of length 16), the received signal is despread using the sequence of the code number, then the despread signal is applied to the preamble sequence detector The despread I and Q channel signals (r The output of the CMF Based upon the output of the max (I) operator Simulation results may be employed to show expected performance of implementations of preamble detectors of While the exemplary embodiments of the present invention have been described with respect to processes of circuits, the present invention is not so limited. As would be apparent to one skilled in the art, various functions of circuit elements may also be implemented in the digital domain as processing steps in a software program. Such software may be employed in, for example, a digital signal processor, micro-controller or general purpose computer. The present invention can be embodied in the form of methods and apparatuses for practicing those methods. The present invention can also be embodied in the form of program code embodied in tangible media, such as floppy diskettes, CD-ROMs, hard drives, or any other machine-readable storage medium, wherein, when the program code is loaded into and executed by a machine, such as a computer, the machine becomes an apparatus for practicing the invention. The present invention can also be embodied in the form of program code, for example, whether stored in a storage medium, loaded into and/or executed by a machine, or transmitted over some transmission medium, such as over electrical wiring or cabling, through fiber optics, or via electromagnetic radiation, wherein, when the program code is loaded into and executed by a machine, such as a computer, the machine becomes an apparatus for practicing the invention. When implemented on a general-purpose processor, the program code segments combine with the processor to provide a unique device that operates analogously to specific logic circuits. It will be further understood that various changes in the details, materials, and arrangements of the parts which have been described and illustrated in order to explain the nature of this invention may be made by those skilled in the art without departing from the principle and scope of the invention as expressed in the following claims. Patent Citations
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